1. Field of the Invention
The present invention relates to a system for personal discrimination and personal identification of a person and, more particularly, to a fingerprint identification system conducting fingerprint identification with higher precision than by conventional systems by using a plurality of fingerprint images at the time of personal discrimination and identification making the use of fingerprints.
2. Description of the Related Art
In recent years, they are well known techniques that personal identification can be made using fingerprints and that an individual can be searched using a large number of fingerprint images which are made into a data base in advance by computer processing or using a feature vector which characterizes the fingerprint images.
In general, such technique is called a fingerprint identification system or an AFIS (Automated Fingerprint Identification System) which is used in identification work by police.
For taking fingerprints of an individual to be personally inquired, it is a conventional practice to imprint his or her fingerprints on paper sheet with ink. Technique has been widespread of taking fingerprint images as digital images directly into a computer by a live fingerprint scanner (or live scanner for short) in real time.
Widely used as a live scanner for taking fingerprint images from fingerprints is a system which takes differences in a light reflected on a prism surface caused by irregularities of ridge lines of fingerprints as variable-density images by means of an optical system.
Various kinds of live fingerprint sensors have been also recently invented and put on the market including a capacitance type fingerprint sensor with numbers of minute electrodes arranged two-dimensionally on a sensor surface for detecting a difference in capacitance generated by irregularities of ridge lines of a fingerprint in contact with the electrode to obtain fingerprint images, a thermal sensor type fingerprint sensor for detecting a difference in temperature caused by the contact between a sensor surface and irregularities of fingerprint ridge lines and an electric field strength type fingerprint sensor for detecting a difference in electric field strength generated by irregularities of fingerprint ridge lines as a result of causing electric field on a finger by electromagnetic wave.
Any of the above-described systems enables fingerprint images to be directly taken into a computer only by brining fingerprints into contact with a live fingerprint sensor surface.
Use of these live fingerprint scanners enables high-speed processing of personal identification using fingerprints.
Also conducted in recent years is to transfer data of fingerprint images taken by means of a fingerprint scanner disposed at a remote place through remote access to a finger print identification sensor via a network and return identification results via the network. While it is possible to send fingerprint images themselves, because images have a large volume of data and because data of ten fingers should be sent for improving identification precision in some cases, a system is also used of sending not an image itself but results obtained by extracting features of fingerprints from an image to a fingerprint identification center (in some cases, images are sent together).
In this case, when sending only a feature vector of fingerprint images, because original fingerprint images exist on the input side, if visual confirmation of the fingerprint images by naked eyes is necessary, the processing will be conducted at a remote place where the fingerprints are input.
On the other hand, use of radio communication has been facilitated nowadays to enable the above-described remote access to be realized by so-called mobile communication.
Under these circumstances, there arises a policeman's need of carrying a mobile communication type fingerprinting device to call on a fingerprint identification center to collate fingerprints on the spot where a person's fingerprints are taken by means of radio communication.
Because a heavily-mounted patrol car is already equipped with radio communication apparatus and also because an information communication terminal using a computer mounted on a car has been widely used, use of a fingerprint identification system in combination with these apparatuses is also demanded.
This enables, for example, criminal histories to be searched and registered personal belongings such as a car to be confirmed using results of fingerprint identification of fingerprint images of a person to be searched.
It is also pointed out that even when an identification score is not sufficient for satisfactorily identifying an individual and, when returning a mug shot of a person whose score is large to a terminal on the spot for deliberate confirmation, this enables a policeman who is not a specialist of fingerprints to identify a person in question with ease.
It is said that such a fingerprint identification system as described above enables detection of a false driver's license, for example.
Results of fingerprint identification is in general represented as an identification score which expresses similarity of fingerprints as a numeric value.
Therefore, when an identification score is extremely large, personal discrimination and identification can be made with high precision even by a single finger.
In order to reduce errors in fingerprint identification, it is as a matter of course possible to check whether fingerprints of a plurality, for example, two including a forefinger and a middle finger or three including the two and a thumb, of fingers of each person all have a large identification score or not.
Even from the same finger of the same person, a large identification score can not be always obtained because of rough skin, injury and fingerprinting being taken out of place and noise. In view of this fact, use of a plurality of different kinds of fingers whose identification processing is possible is useful.
At a patrol spot, it is assumed that when taking fingerprints from a suspicious person, he or she might not be cooperative.
It is also pointed out that in a case where general policemen use the system, even if they are trained to take fingerprints, they will have difficulties in taking as high quality fingerprint images as those taken by specialists in identification.
In other words, demanded is a remote-access type or in particular, mobile communication type fingerprint identification system enabling everybody to take as high quality fingerprint images as possible with relative ease to realize fingerprint identification with high precision using the taken fingerprint images although training for operating the system is required to some extent.
On the other hand, in order to obtain identification results of high quality fingerprint images, as to the use of a plurality of fingerprint images of the same finger, proposals are made in two aspects.
First is taking a plurality of images into a processing device in order to determine, when an imprinting area is increased in time series at the fingerprinting on a live fingerprint sensor, whether the imprinting area becomes a sufficient area.
In this case, since clear images with excellent contrast can not be obtained unless an area and imprinting pressure are sufficient, a mean density, variance, a maximum value, a difference between a maximum value and a minimum value and the like are often used as statistic property of each pixel within a screen.
Moreover, for example, Japanese Unexamined Patent Publication (Kokai) No. Heisei 1-217574, entitled “Device for Collating Principal” obtains stable fingerprint images without blur by the provision of a means for storing a plurality of so-called frame image series succeeding one after another in time series and determining whether a finger has moved or not and a means for presenting alarm when it is determined that the finger is moving.
In Japanese Patent No. 2680084 entitled “Personal Identification Device”, aiming at obtaining a stable feature vector, at the time of registration to a fingerprint DB (data base), a plurality of fingerprints are imprinted with a feature point whose occurrence frequency is high weighed.
Also here, a plurality of images are taken from the same finger, which is the arrangement at the time of generating registered images that is made regarding a feature point occurring in every frame in time series as being important.
These conventional techniques are all premised on that one high quality image or one set of feature vectors equivalent to the same is ultimately extracted.
Second aspect relates to identification precision.
Commonly used as criteria for comparing so-called identification precision of patterns in a case of fingerprint identification have two kinds, a false match rate (FMR) which is a rate of determining that a fingerprint image of other finger is that of the same finger and a false non match rate (FNMR) of determining that a fingerprint image of the same finger is that of other finger.
Here, it is known and important that an FMR and an FNMR have a trade-off relationship.
More specifically, setting for improving an FMR (reducing FMR) in order to exclude erroneous acceptance of others conversely leads to an increase in an FNMR of determining that a person in question is other person.
At present there exists a system for ensuring security by using fingerprint identification, which conducts, for example, user authentication at opening and closing of a door and computer log-in by fingerprint identification.
In a case where such a system as described above with low identification precision is used, however, setting the FMR to be low so as to increase probability that other person will be erroneously accepted as a registered person results in an increase in the FNMR as a by-product to make even a person in question be hardly accepted as he/she is.
Known technique as a countermeasure is imprinting a plurality of fingerprints of the same finger and when at least one of the fingerprints has a large identification score, passing a person in question.
In this case, the countermeasure is characterized in using an identification score of a plurality of fingerprint images of the same finger based on the fact that “at least one of a plurality of images has a large identification score”.
Conventional art thus aims at taking one fingerprint image of highest quality according to the first aspect.
This is thought to take communication costs and feature extraction processing costs into consideration.
However, recent increase in a band width of communication paths, possibility of large volume of data communication and realization of high-speed processing and parallel processing at low costs because of development of semiconductor techniques make constraint of “only one highest quality image” be not always necessary.
According to the second aspect, conventional art requires at least one high-quality image usable among a plurality of images.
In this case, however, it is constrained that at least one of them should have very high quality and a large identification score.
Therefore, as described above, there are needs of a technique of taking high-quality fingerprint images and an identification method which facilitate fingerprinting even when a person is not cooperative in taking his/her fingerprint images and which can be used with simple training.